Amplitude limiting circuit arrangement



w. BEUKEMA 2,616,967

AMPLITUDE LIMITING CIRCUIT ARRANGEMENT Nov. 4, 1952 Filed March 8, 1950 INVENTOR. WILLEM BEUKEMA AGEN Patented Nov. 4, 1952 iAMPLiTUnE LI ITING CIRCUIT ARRANGEMENT "Willem Beukema, Eindhoven, -Netherlands,- as- -signor;; to: Hartford N ational Bank and :Trust f. Company, Hartford, Conn., l as trustee Application-March 891950, SerialNo. 148,422 ln the' Neth'erlands MarcH 10,1949 Claims. (01. 178-144) Thepresent :invention' relates, to circuits for ,l limiting tha -amplitude of ielectrical oscillations, comprising' an amplifyin tube, the control-(grid circuit of which includes acircuit tuned to the ,Lfrequency of the oscillations to be limited and the coutputcircuitof which includes anamplitude .detector and the output electrodes for derivingthe limitedoscillations.

.In ;a known circuit of this kind, the amplitude detector is connected 'insuch manner that, if' the v oscillations, generated inithe output circuit of .the amplifying. tube have-a comparatively large-amplitude, a' control voltage is-produced I in the output circuit of the amplitude detector. 7 This con- ,-trol voltage reduces the mutual conductance of the amplifying vtube,.compensating for the firstmentioned increase in amplitude.

In a furtherlmown circuit,the' oscillations to ,..be limited are supplied to an: amplifying tube, the v,gri d circuit of which includes a'condenser .and'a resistance across which appears a control voltage. This control voltage also'reduces the tube mutual conductance at comparatively high inputampli- I The circuit accordingto the, inventioncisubased on a different principle which: allowsi a greater suppression of; unwanted amplitude -modulation. It. ischaracterized in that the output voltage of the amplitude detector is supplied to the controlgrid circuit ofthe amplifying tube in in-phase .relationship, so that an increase in amplitude of the input oscillation results ingreatercontrolgrid currentand hence greater damping of the tuned circuit.

The invention will now be explained more fully byreference to the accompanying drawingf'in which:

Figure 1 shows a schematic diagram Mahmiter circuit according to the invention, and

Figure 2 shows a further embodiment of the circuit illustrated in Fig. 1.

Referring now to the drawing, and more particularly to Fig. 1, the oscillations to be limited, which may be, for example, frequency-modulated oscillations having unwanted amplitude modulation components, are produced in the output circult of? an 5 amplifying tube l "and are applied to the input circuit 10f an--:.amplifying tubeyZ. :The ccontrol=grid-fcircuit of tube 2.includes;a.cir c u-it-3 which is tuned to the" frequencyi'of 'the incoming oscillations. A

v The oscillations developed intheoutputrcircuit of theamplifying-tube 2' are' suppliedrto: anamplitu de detector which comprises a rectifier 5 andan output filter'B. Thetime-constant 'of filter 6 is shorter than the: period.'ofjthetrhighest audio-frequency to be transmitted. ,Aifilterfifl,

which is included in the cathodefcircuit'ioftube 2, provides a negative grid bias for: tube :2.

p According to the invention, ther'ectifierrzli: has

a unidirectional characteristic suchilthatiupon an increase in' amplitude'rofithe output::osc illa 'tion of tube 2, a'higher positive voltageiis produced; across the filter. ,6. :This 'voltage ibrings about a "decreaseini the negative'grid bias of tube' 2. I As 'a result, 1 a greater contriole'grid :"current' starts-to flow in this 'tube, fbrin'ging about: greater damping or the tuned circuit 3. The oscillations produced'at -a pair of output terminals kwhich terminals are coupled to "the 1 output circuit of tube 2, thus exhibit: a; substantially constant-amplitude.

In contradistinction with the first-mentioned known circuit," the control" grid of tube *2 is ad justecl to arless rnegative" value upon-increasein amplitude of "the output oscillation,"while iri' tt'ie known circuit :thesaidx grid. is ;':"as: a riulei adjusted ".toia more negativewvalue. The: circuitzarrangement ac'cording'to the invention exhibits 'g'r'eater stability i than the iknown circuits because it i is not: necessary "to employ 1' networks or amplifiers 'between 'theiifilter 6 ":a-nd 'ithe"'controli-grid iof i'tubeil.

,: The'circuit' exhibits r a sensitivity hi'gher than "that oft-the second known' "circuit beo au'se a' "high control-voltage is: available for producing gridJ-curre'nt to damp circuit 3.

control- In order 'to bring aboutgreater-suppression of unwanted amplitude modiilation a second filter 1', which' has a time-constant -lo nger than the period of the lowest audio-frequency to' be suppressed, may be includediri the circuit ot-the ainplitude detector 5 6. Across the filter I "there appears avol-tage equal to the mean ampuwdecr the oscillations "set up at the output electrodes 4. A momentary increase in amplitude of the said oscillations will thus cause a considerable current to flow through the rectifier 5, so that this increase in amplitude will be suppressed. The voltage set up across the filter 1 may further be used for automatic gain control.

Fig. 2 shows a circuit in which additional suppression of unwanted amplitude modulation may alternatively be ensured with the use of a rectifier 5 having a comparatively high internal resistance. For this purpose, the output circuit of the amplifying tube 2 includes an impedance-inverting network It constituted, for example by two tuned circuits I I, I2, which circuits are overcritically coupled. The output of network II] is supplied to the amplitude detector 5, 6. As in Fig. 1, the network 6 has a time-constant shorter than the period of the highest modulation frequency to be suppressed, so that here again an oscillation is produced corresponding to the unwanted amplitude modulation. The amplitude detector 5, 6 thus constitutes an impedance which tends to suppress the amplitude modulation of the current traversing this impedance, so that the amplitude modulation of the voltage is suppressed at the input of the impedance-inverting network In. This method has been extensively described in the former application Ser. No. 727,443, filed February 8, 1947 by Jacob van Slooten, and having a common assignee with this application.

While I have described my invention in a specific use thereof and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the alt without departing from the spirit and scope of the invention.

What I claim is:

1. An electrical circuit-arrangement for limiting amplitude modulation components of an applied signal, comprising an electron discharge tube having a cathode, a control grid and an output electrode, an input circuit having a given damping and tuned to the frequency of said signal coupled to said control grid, an amplitude detector circuit coupled to the said output electrode, means to derive a control voltage from said detector and to apply said control voltage to said control grid in regenerative relationship to produce grid current fiow in said discharge tube thereby to increase the damping of said input circuit as the amplitude of the applied signal increases.

2. An electrical circuit arrangement for limiting amplitude modulation components of an applied signal, comprising an electron discharge tube having a cathode, a control grid and an output electrode, an'input circuit having a given damping and tuned to the frequency of said signal coupled tosaid control grid, an amplitude detector circuit coupled to said output electrode and comprising a rectifier element and a filter network having a time constant shorter than the minimum period of said modulation components, means to derive a control voltage from said filter and to apply said control voltage to said control grid in regenerative relationship to produce grid current flow in said discharge tube thereby to damping and tuned to the frequency of said signal coupled to said control grid, an amplitude detector circuit coupled to said output electrode and comprising a rectifier element and a first filter network having a time constant shorter than the minimum period of said modulation components, a load circuit coupled in parallel with said amplitude detector circuit and including a second filter network having a time constant longer than the maximum period of said modulation components, means to derive a control voltage from said first filter and to apply said control voltage to said control grid in regenerative relationship to produce grid current fiow in said discharge tube thereby to increase the damping of said input circuit as the amplitude of the applied signal increases.

4. An electrical circuit arrangement for limiting amplitude modulation components of an applied signal, comprising an electron discharge tube having a cathode, a control grid and an output electrode, an input circuit having a given damping and tuned to the frequency of said signal coupled to said control grid, an impedance inverting network coupled to said output electrode, an amplitude decteotor circuit coupled to said impedance inverting network and comprising a rectifier element and a filter network having a time constant smaller than the minimum period of said modulation components, means to derive a control voltage from said filter and to apply said control voltage to said control grid in regenerative relationship to produce grid current flow in said discharge tube thereby to increase the damping of said input circuit as the amplitude of the applied signal increases.

5. An electrical circuit arrangement for limiting amplitude modulation components of an applied signal, comprising an electron discharge tube having a cathode, a control grid and an output electrode, an input circuit having a given damping and tuned to the frequency of said signal coupled to said control grid, an impedance inverting network comprising a pair of overcritically coupled resonant circuits coupled to said output electrode, an amplitude detector circuit coupled to said impedance inverting network and comprising a rectifier element and a filter net-- work having a time constant smaller than-the minimum period of said modulation components, means to derive a control voltage from said filter and to apply said control voltage to said control grid in regenerative relationship to produce grid current flow in said discharge tube thereby to increase the damping of said input circuit as the amplitude of the applied signal increases.

WILLEM BEUKEMA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,395,615 Curtis Feb. 26, 1946 2,512,658 Levy June 27, 1950 2,524,556 Worchester, Jr Oct. 3, 1950 

